This invention relates to an integrated telecommunications system, and more particularly a system comprising wired and wireless subsystems.
PBX-base wired telecommunications systems, for example telephone systems, have been established in business premises for many years. These are now largely implemented in digital form using well-established technology. More recently, in order to provide additional mobility it has become desirable to associate these conventional wired systems with cellular wireless systems, usually employing RF links, that permit the users to carry portable hand sets on their person. Just as in the case of large scale commercial cellular systems, these local cellular systems employ RF base stations distributed throughout the premises and controlled so as to establish links with selected handsets in accordance with signal strengths. As the user moves around the building, a central controller ensures hand-off from one base station to another. Control of the system is accomplished by messages passed through the RF links from the hand sets to a controller via the base station and vice versa.
The base stations are placed around the building as a shared resource. It is clearly desirable to keep the number and complexity of base stations to a minimum in order to reduce overhead costs. Unfortunately, the RF medium is unstable. Environmental changes occur such that even a stationary terminal cannot be guaranteed a reliable connection to any specific base station. Fading may occur due to subtle environmental changes unrelated to any movement on the part of the user. This can necessitate hand-off from one base station to another or between different channels associated with the same base station. Of course, as the user moves about the building, hand-off becomes necessary as different base stations come into range.
The cellular subsystem must guarantee connection at all times to one base station, and hand-off must be imperceptible to the user. This requires that hand-off be done very quickly so that interruptions in service are limited to less than a few tens of milliseconds.
The control of the wireless subsystem to maintain reliable connections at all times to a base station is generally known as mobility management. There are various wireless protocols that support hand-off, but generally the hand sets are provided with connections on two different RF channels. One channel is used as the active channel with the other on standby in the event that the signal quality on the first channel degrades beyond a predetermined satisfactory level. These two channels can establish connections with different base stations or represent different RF channels associated with the same base station. The two channels should be selected in such a way that their characteristics differ sufficiently for simultaneous degradation on both channels to be unlikely so as to ensure the provision of satisfactory service.
A serious problem in wireless systems is known as a hard fade. Slight changes in the environment can cause the attenuation of a RF channel to vary by up to 100dB for practical purposes, instantaneously (in the order of microseconds) which is several orders of magnitude greater than the tens or hundreds of milliseconds representing the response time of a typical communication system. For a wireless system to provide a service comparable to that of a wired system, channel switching must occur sufficiently fast that no perceptible degradation will occur in the event of a hard fade. For practical purposes, in the case of speech this means that the switch must occur in less than 50 to 100 milliseconds if an unacceptable pop is not to be heard on the connection. Synchronization must be restored with an average maximum time of less than 50 ms for adequate user performance.
The 50 ms requirement is much too fast for a conventional communication system, such as a PBX. These systems, which share resources among users with various types of messaging and queuing schemes, cannot respond faster than around 500 ms for conventional signals, such as occur when a terminal goes off hook. Hand-offs generate far more messages, which require much faster response times.
Even if the problem of response time is overcome, users still expect wireless systems to offer all the features of a fully featured PBX. These include user-based features such as call forwarding and the ability to interface with LAN and ISDN systems. The user should be able to make a connection and receive any service in the wireless environment that can be received through the wired system. Existing wireless systems usually employ wireless controllers connected to the PBX through ONS line circuits or some form of high speed digital trunk. The PBX controls the wireless extensions in the same manner as an ordinary extension, with the wireless controller taking care of mobility. The Ericsson DCT900 and the Novatel Affinity are examples of such products. These treat the wireless system as a separate device connected by trunks. Access to the feature base is extremely limited since the features can be supplied only by trunk protocols of varying capability.
It has been proposed to isolate the PBX from the wireless control system since these domains have different requirements and require different solutions. However, the provision of a physically separate system for the wireless subsystem results in loss of flexibility of an integrated system and requires a significant degree of wiring duplication since the same physical wiring can no longer be shared. The provision of separate systems also results in the loss of the close degree of coupling possible between the wireless side and other subsystems that can be offered by an integrated system.
It has been suggested that base stations could be formed into groups using RF combiners. British Telecom has suggested a scheme whereby the RF from all the base stations is combined and carried back to a common pool of receivers at a central controller from each base station by a modulated optic fiber carrier. The optic fiber transports the RF from each base station back to a common RF selector.
RF based systems have a number of disadvantages. They do not allow different base stations to use the same frequencies since they have one set of receivers for all the RF channels in a group.
Such existing systems are only useful to voice only wireless subsystems since they cannot provide solutions similar problems in an integrated communications systems, such as LAN connectivity. The development costs of supporting these wireless applications on the PBX cannot support other applications. Furthermore, an integrated communications system should provide close coupling among all applications. For example, a user might wish to connect a wireless terminal to a LAN. The RF macro-cell does not assist as a solution to this problem.